Spontaneous plaque forming cells in the peripheral blood of patients with systemic lupus erythematosus.

A reverse haemolytic plaque assay using staphylococcal protein A coupled to sheep red blood cells was set up in Cunningham chambers. Using this method, the numbers of Ficoll-Hypaque isolated peripheral blood lymphocytes (PBL) secreting IgG, IgA or IgM without preceding culture or mitogen stimulation were estimated in patients with systemic lupus erythematosus (SLE) and control subjects. Seven patients with clinically inactive SLE at the time of the study had values similar to those of the control subjects. In contrast, eight patients who had clinically active SLE had markedly increased numbers of PBL secreting IgG, IgA and IgM. Control experiments confirmed that the plaques were due to Ig secretion by lymphoid cells rather than to immune complexes adsorbed onto Fc receptor bearing cells or to passively adsorbed Ig. The results confirm the expected polyclonal B cell activation in patients with SLE and serial measurements showed that clinical relapses occurred only when the numbers of immunoglobulin secreting cells were high. Experiments in three patients with active SLE using native DNA prepared from T2 bacteriophage as the 'developing antigen' suggest that PBL secreting nDNA antibody can also be demonstrated by this method.